[amsat-bb] Re: Turn off AGC when receiving BPSK-1000
Mark L. Hammond
marklhammond at gmail.com
Wed Aug 17 13:36:09 PDT 2011
This is a great reminder. Thus far, my data has been collected with the TS-2000x using a "mid" setting for AGC; and with the HDSDR software set to "AGC Med" when using/playing back Funcube Dongle data. I've set both to OFF now, since it's possible
I wonder if anybody has experimented with AGC settings and the HDSDR decoding? Might be worth running a recording through a few times at various settings..hrm.
In any event, Phil...THANK YOU for making this code real. I have seen it print data when the signal was visibly "in the dirt" which is impressive and fun to see.
At 02:31 PM 8/16/2011 -0700, Phil Karn wrote:
>I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000
>telemetry beacon: turn off your receiver AGC if at all possible. If you
>can only choose between fast and slow, pick slow. If this causes a large
>variation in audio level, reduce the gain to avoid clipping on the
>peaks. A sound card A/D is 16 bits so you have plenty of dynamic range;
>don't be afraid to use it.
>Ideally the background noise level should be constant with the signal
>going up and down.
>This greatly helps the demodulator and decoder to distinguish signal
>from noise. The error correction uses the Viterbi algorithm, and one of
>its big features is the ability to distinguish between "strong" and
>"weak" bits; a strong '1' or '0' is considered less likely to be in
>error than a weak '1' or '0'. The decoder can even accept "I don't know"
>for a limited number of bits.
>The decoder can still fix errors in strong bits. But it can fix more of
>them in the weak bits and still more in the "I don't knows" (known
>technically as "erasures").
>This is especially important when the signal fades deeply, as it often
>does with ARISSat-1. With the AGC off, the audio signal level falls
>during a fade and the decoder can recognize it as a burst of erasures or
>As with many questions in life, "I don't know" or "I think it's X but
>I'm not sure" are better answers than being sure of the wrong answer.
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